We propose to define those active ion transport processes of the small intestine that produce osmotic work (absorption and secretion of fluid) and to identify the intra- and extracellular substances (cyclic nucleotides, calcium, tubulin, hormones, neurotransmitters) which regulate these processes. Through measurements of (1) radioisotope fluxes (Na, Cl, Ca) into and across the epithelium and in and out of isolated epithelial cells, (2) transepithelial PD and resistance, (3) cyclic nucleotide concentrations in isolated epithelial cells and (4) interactions with hormone receptors on isolated epithelial cells, we hope to gain insight into the relationships among extracellular stimuli, intracellular mediators and active ion transport and into the specific roles of villus cells as opposed to crypt cells and of the basolateral membrane as opposed to the brush border membrane. Studies will be carried out on both mammalian and teleost intestine and on isolated tubules from shark rectal gland.